1. Field of the Invention
The present invention relates to a control system for controlling a hybrid vehicle having an internal combustion engine and an electric motor as separate propulsion sources.
2. Description of the Related Art
There have heretofore been known hybrid vehicles each having an internal combustion engine and an electric motor as separate propulsion sources. One known control system for controlling the propulsion sources of such a hybrid vehicle is a control system mounted on a large-size bus as disclosed in Japanese laid-open patent publication No. 3-121928.
The disclosed control system, which controls a hybrid vehicle having a diesel engine and an electric motor, determines an assistive power to be generated by the electric motor based on the rotational speed NE of the diesel engine and the position of a control rack of a fuel supply pump. The control system controls the electric motor to generate an assistive power for the diesel engine when the diesel engine is subject to a high load range higher than a predetermined level. The diesel engine is thus operated only in a load range smaller than a full load range for the purpose of improving engine exhaust gases containing nitrogen oxides and black smoke. The control system primarily controls the electric motor to generate an assistive power when the large-size bus starts to move, posing a high load on the diesel engine. However, it is preferable for the electric motor to generate an assistive power depending on the propulsive force that is needed by the hybrid vehicle even when the hybrid vehicle is normally accelerated. Since the conventional control system does not pay much attention to the generation of an assistive power from the electric motor when the hybrid vehicle is further accelerated while the diesel engine is rotating at a high speed or the hybrid vehicle is running at a high speed, the conventional control system suffers drawbacks as described below.
When the hybrid vehicle is to be accelerated while it is running at a high speed, the driver tends to press the accelerator pedal deeply though it does not intend to accelerate the hybrid vehicle so rapidly. Therefore, the control system may control the electric motor to generate an assistive power even if such an assistive power generated by the electric motor is not in fact required. When this happens, a certain amount of stored electric energy is wasted by the electric motor, and it may be too large to energize the electric motor to generate a sufficient assistive power that may be necessary next time to accelerate the hybrid vehicle from a low speed. When the hybrid vehicle is running at a high speed or the diesel engine is rotating at a high speed, since the resistance to the running of the hybrid vehicle (running resistance) and the counterelectromotive force of the electric motor are large, a large amount of stored electric energy is needed to energize the electric motor to generate an assistive power.